Intravenous infusion sets commonly include drip chambers (DCs) to enable the flow rate of an infusion liquid to be visually observed. In a typical installation, an infusion bag is suspended above a patient and a spike at an inlet of the DC pierces the bag, whereby the infusion liquid drips into the DC. The flow rate of the infusion liquid may be observed by the rate of formation of drops of the liquid from the inlet into the DC down through an outlet of the DC which is connected to tubing to the patient. The DC helps prevent air from entering the downstream tubing. This is achieved by a liquid layer acting as an air barrier at the bottom of the chamber. It is formed by manual “priming” (squeezing) of the chamber before the start of infusion.
Commercial drip chambers are constructed simply of a flexible, transparent cylinder with inlet and outlet. They also often contain a screen at the bottom, acting as a coarse filter preventing particles from entering the vein.
It is desirable to have a finer filter such as a hydrophilic membrane, ideally a bacteria retentive membrane, incorporated into the DC. This is advantageous in that it obviates the use of an extra filter often connected downstream of the infusion set. More importantly, such a membrane has the advantage of solving the following problem often encountered in infusion. When the infusion bag empties, so does the DC. As the above mentioned liquid layer is now absent, air enters the tubing downstream of the DC. This necessitates opening the system and re-priming the set before infusion can be continued. By using a hydrophilic membrane at the DC outlet, liquid flow is not impeded, yet air intrusion is substantially prevented at all pressures below the membrane “bubble point”. Thus after the infusion bag empties a new bag can be connected and infusion restarted without need for re-priming.
A few patents (e.g., U.S. Pat. Nos. 4,013,072, 4,521,212 and 5,902,281, the disclosures of which are incorporated herein by reference) have described infusion devices that include membranes; however no simple, membrane-based infusion set is available commercially. The reasons for this are both technological and commercial. Membranes are sensitive structures and may easily be affected by solvents, glues and other bonding means. Also infusion sets need to conform to certain flow and accuracy criteria which may be affected by the membrane. Additionally the infusion set must remain inexpensive even with the membrane incorporated.
Another infusion device with a membrane is described in U.S. Pat. No. 5,779,674 to Ford. In this device, a hydrophilic membrane is attached to a support structure and may be horizontally or vertically oriented. However, this device has a drawback of the membrane being situated close to a base which is bonded with an adhesive to a drip chamber, meaning that the membrane can possible come into contact with the adhesive. In addition, the structure of the membrane support is such that air entrapped downstream of the membrane between the membrane and drip chamber outlet, can occasionally escape into the infusion line, thus defeating the whole purpose of the membrane drip chamber.
The need therefore exists for an economical, accurate, high throughput, membrane-containing drip chamber that poses no danger of air escaping into the infusion line.